Double hung windows can be opened for ventilation from the top or bottom. While most windows have locks which are operable when the windows are closed, a partially open window is often left without recourse to locking. Some windows have built-in restraints to permit locking to prevent opening more than a certain fixed amount. Auxiliary screw operated locks which must be screwed into window frames are also well known, but rental apartments often disallow such modifications. An adhesively-mounted removable retrofit adjustable restraining stop for double hung windows is not known in the prior art.